Because in the field of electronics in general further downscaling of the semiconductor devices is always proceeding, deposition processes have to be developed that are able to deposit layers with a thickness control at atomic layer scale. One of these deposition techniques is Atomic Layer Deposition (ALD), often used for depositing dielectric layers.
In order to achieve satisfying semiconductor device characteristics the interface quality between the semiconductor substrate and the ALD layer has to be controlled sufficiently.
Since for ALD processes the deposition is very temperature sensitive, this control of the interface quality is very challenging.
One of the main reasons why ALD is temperature sensitive is that depending on temperature the semiconductor substrate always has a limited concentration of [OH] (hydroxyls) groups at the surface, which are the active sites for starting the ALD reaction cycle.
Methods have been described intended for preparation of silicon surfaces with a concentration of [OH] at 5×1014 cm−2 before starting the HfCl4 ALD reaction cycle. If each of these reactive sites were utilized one could expect good interface formation. However, data from electrical evaluation of devices with high-k grown with a standard ALD process consequent to these surface preparation methods indicates poor interface quality.
As mentioned above, it is known that the concentration of surface hydroxyls decreases with increasing temperature. Since the semiconductor substrate surface is at a temperature of 300° C. during the conventional ALD deposition the [OH] concentration is limited.
At lower temperature the silicon surface has a significantly higher concentration of surface hydroxyls than under the conventional deposition temperature of 300° C. However, the lower temperature has the negative side effect of not driving the ALD reaction cycle to completion.